Devices using various types of media such as optical disks, magnetic optical disks, and a flexible magnetic disk, are known in the art as disk drives. Among them, hard disk drives (hereinafter referred to as HDDs) have become popular as storage devices for computers to such an extent that they are one of the storage devices indispensable for modern computer systems. Further, not limited to the computers as described above, HDDs are expanding more and more in application due to their excellent properties. For example, HDDs are used for moving picture recording/reproducing devices, car navigation systems, cellular phones, and removable memories for use in digital cameras.
The magnetic disks used in the HDD each have a plurality of concentrically formed tracks, and each of the tracks is subdivided into a plurality of sectors. Address information on the sector, and user data are stored into each sector. A read/write head can access a desired sector in accordance with the address information of the sector, thus writing data onto or reading out data from the sector.
Before being transmitted to a host, signals that the head has read out from the magnetic disk in a data readout process undergo required signal processing by a finite impulse response (FIR) filter, such as wave shaping and decoding. Typically, the signals from the head are waveform-equalized by the filter and after being converted into digital signals by an A/D converter, these signals are once gain subjected to waveform equalization by the waveform equalizer. Additionally, signals from the waveform equalizer are decoded and demodulated.
In recent years, a perpendicular magnetic recording scheme has been researched as a method of recording on a magnetic disk. The perpendicular magnetic recording scheme allows a stable recording state to be maintained at high recording density as compared with conventional longitudinal (planar) magnetic recording. Perpendicular recording is known as a scheme in which the low-frequency components of data signals contribute to the improvement of error rates. It is possible to prevent the degradation in error rate of perpendicular recording, that is, to obtain an error rate reduction effect, by assigning a low cutoff frequency (low pole frequency) to the high-pass filter of a preamplifier and supplying signals of a frequency equal to or higher than a required value to a PRML (Partial Response Maximum Likelihood) system Viterbi decoder for a read channel.
During the reading of data from a magnetic disk, thermal energy may be generated due to a possible collision between a head and the magnetic disk, causing thermal asperity (TA), which is an event that magnetic resistance changes and the a read signal significantly fluctuates in DC level. If a DC offset of the read signal occurs due to the thermal asperity, the read signal must be immediately returned to its original DC level. Accordingly, if the thermal asperity occurs, an error recovery process has been traditionally performed, which increases the cutoff frequency of the high-pass filter to several megahertz.
In the perpendicular recording scheme, however, the frequency used under normal operating conditions is several hundreds of hertz. Thus, if the cutoff frequency increases to several megahertz during the error recovery process, the frequency significantly changes, thereby extending the time required for the read signal to recover the original DC level. This is because the optimum values for normal error recovery are preassigned as tap coefficients of a transversal filter in the perpendicular recording scheme. That is to say, if TA occurs, the tap coefficients of the transversal filter depart from the respective optimum values for normal error recovery.
This causes the degradation in error rate. For this reason, the use of the perpendicular recording scheme results in the contradiction that the process for error recovery causes the degradation in error rate. If TA occurs, therefore, an error recovery effect cannot be obtained just by increasing the cutoff frequency of the high-pass filter.
For the above problem, there is a method of converging noise by optimizing the tap coefficients of the transversal filter after increasing the cutoff frequency of the high-pass filter in the event of TA. However, the transversal filter, which is to be applied to the PRML system Viterbi decoder, needs to have a high order of magnitude in the number of taps, for example, 16 taps.
In the meantime, calibration becomes a time-consuming task since a large number of data sectors must be read to optimize the tap coefficients stably. As can be understood from these facts, the following two processes cannot be realized simultaneously: optimizing all tap coefficients and converging noise quickly for error recovery within a minimum time. In other words, optimizing all taps requires slow feedback and is likely to result in system divergence if feedback is accelerated.
In the conventional longitudinal magnetic recording scheme, since the frequency used under normal operating conditions is several megahertz, even after the cutoff frequency has been increased as the recovery process associated with TA, a variation in the operating frequency is sufficiently suppressed and the time required for the recovery to the original DC level is not extended.
For these reasons, Japanese Patent Laid-Open No. 2004-326973 (“Patent Document 1”) discloses a magnetic disk drive using a perpendicular magnetic recording method with reduced effects caused by TA. The magnetic disk drive described in Patent Document 1 has dual-system data detectors each having a different cutoff frequency. In the drive, the appropriate data detector is selected, depending on whether TA has occurred. That is to say, when TA is detected, a read signal for data which has been written in the perpendicular recording scheme is differentiated before being subjected to data detection.
For Patent Document 1, however, when the cutoff frequency is changed in the event of TA, since the read signal is differentiated to suppress the effects of the DC component, a detection target is also changed, which results in the necessity for changing each tap coefficient as well. Thus, two different types of parameter set are necessary: a parameter set for normal conditions and a parameter set for TA. This requires a large amount of memory capacity.